stella programmer's guide
The "Stella Programmer's Guide", originally written in the late 1970s, is considered by many Atari 2600/VCS homebrew developers to be 'the original bible' of Atari 2600 development. This HTML version is somewhat more convenient than the original plaintext version in that it provides links between related terms in the guide in order to make it easier to navigate.
Note that this isn't really a step-by-step tutorial on how to get started on Atari 2600 development (those are available elsewhere online), but it does serve as a good introduction to the system as well as a handy reference guide once you've gotten started.
entex adventure vision 'hello world' demo
The Entex Adventure Vision is an obscure 80s videogame system that only ever had 4 commercial games released for it. This writeup explains the system architecture in some detail and then walks through a simple 'hello world' demo written for it in 8048 assembly. A link to the source code is provided at the end.